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NAD C268 Power Amplifier Review

Ivaols

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#62
I use NAD C268 to drive a couple of 111db sensitive compression drivers. The reason for picking up NAD C268 was XLR inputs, but mainly the gain adjustment. Low gain in the amplifier has proved to be the key in my system for preventing hiss noise from the speakers. I am curious to know if reducing the gain will improve SINAD on this amp?
 
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#64
I see a SNR of 90dB is good here. But when I look at manufacturers spec of many amps they all have more than 100 dB.. some vintage amps claim 110dB. So are they all false claims or on a special occasion they hit value ? Like peak power?
For the NAD C268, the spec sheet (https://nadelectronics.com/wp-content/uploads/2017/10/NAD-C-268-Data-Sheet.pdf) lists under ANALOG AUDIO INPUT/LINE OUT, Signal-to-Noise Ratio >110 dB (IHF; 20 Hz – 20 kHz, ref. 2V out). Under ANALOG AUDIO INPUT/SPEAKER OUT, it lists Signal-to-Noise Ratio >98 dB (A-weighted, 500 mV input, ref. 1 W out in 8 Ohms).

Amir states "Here is our usual dashboard where we feed the amplifier a simple 1 kHz sine wave/tone and ask it to produce 5 watts into 4 ohm load (typical for many speakers). This is what we get:"

Then he lists SNR ~95 dB at 5 watts, 107+ at full power.

For reference, the top-rated SINAD amplifier at ASR seems to be the Benchmark AHB2 (https://www.audiosciencereview.com/...and-measurements-of-hypex-nc400-diy-amp.5907/), which has a SNR of 120 dB "Using near max power."

I wonder if there is a a reason for difference in specifications between 1 watt and 5 watts and full power (or 2V out), also 8 ohms and 4 ohms?
 
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#65
I see that you have Klipsch Forte III speakers which have a sensitivity of 99dB and are rated for 116dB continuous output. 20 years ago, I could drive my old Forte II's to ear-splitting levels with a 3wpc single-ended 2A3 tube amp.

I do believe that your Audiophonics amplifier with its NCore NC250MP modules is not likely to reach clipping levels in your system! :rolleyes:
I don't really believe that sensitivity rating of the Forte's. My previous B&W's were not very sensitive and there is only a small difference in volume settings. Headroom is the name of the game.

I see a lot of people drive the Forte with tubes. Any particular reason you do? I'd get it for any other Klipsch speaker but the Forte is the only Klipsch I know of that isn't shouty or harsh out of the box. I won't praise their sound quality but they are good fun. Could have been better built for the money.
 
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#66
mostly kits - some like DIY others don't
Plenty of small companies will build them for you. Audiophonics, Rouge, KJF, Nord, Apollon, VTV and IOM were what I took into consideration but there is bound to be more. I wanted a 12v trigger input, EMI/RF rejection on the power inlet and shipping from inside the EU and it was very easy to find a good feal among them.
 

Xulonn

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#67
I see a lot of people drive the Forte with tubes. Any particular reason you do?
I owned a second, larger system with the Forte II's for the 2.5 years (2003-2005) that I lived with my late partner in her house with a large, high ceiling (15 peaked ceiling) prior to her passing. It was mainly a sentimental purchase based on memories of my first tube amplifier + big bass reflex mono system while still in high school in 1958. I still owned my older Bryston integrated + Apogee Centaurus Ribbon monitor system - in the dining room - and returned it to duty as my "main" system after my partner's family sold her house, and I moved into a much smaller apartment.

The Bryston/Apogee system was certainly more "accurate" - but I enjoyed both. Like most big efficient speakers, the Forte II's played up to high volumes with a sense of ease. It may have been a psychological illusion, but big ported or passive radiator systems always have that effect on me, whereas inefficient speakers with monster amplifiers often seemed to be working hard to create that same sense of music flowing easily.
 

sejarzo

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#68
I wonder if there is a a reason for difference in specifications between 1 watt and 5 watts and full power (or 2V out), also 8 ohms and 4 ohms?
Someone correct me if I am wrong, but the random noise/hum/etc. that many components in the circuit produce is present even with no signal. It's the same amount of voltage, regardless of output level...so that noise is a higher percentage of the total output when the output power is low, and a lower percentage when the total output is near maximum power.
 
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amirm

amirm

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Thread Starter #69
Someone correct me if I am wrong, but the random noise/hum/etc. that many components in the circuit produce is present even with no signal. It's the same amount of voltage, regardless of output level...so that noise is a higher percentage of the total output when the output power is low, and a lower percentage when the total output is near maximum power.
That's correct. This is the reason I measure at 5 watts than 1. Otherwise "distortion+noise" would just be noise.

Today's amplifiers produce far more power anyway so 5 watts today is what 1 watt was back in 1960s....
 

pjug

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#71
For the NAD C268, the spec sheet (https://nadelectronics.com/wp-content/uploads/2017/10/NAD-C-268-Data-Sheet.pdf) lists under ANALOG AUDIO INPUT/LINE OUT, Signal-to-Noise Ratio >110 dB (IHF; 20 Hz – 20 kHz, ref. 2V out). Under ANALOG AUDIO INPUT/SPEAKER OUT, it lists Signal-to-Noise Ratio >98 dB (A-weighted, 500 mV input, ref. 1 W out in 8 Ohms).

Amir states "Here is our usual dashboard where we feed the amplifier a simple 1 kHz sine wave/tone and ask it to produce 5 watts into 4 ohm load (typical for many speakers). This is what we get:"

Then he lists SNR ~95 dB at 5 watts, 107+ at full power.

For reference, the top-rated SINAD amplifier at ASR seems to be the Benchmark AHB2 (https://www.audiosciencereview.com/...and-measurements-of-hypex-nc400-diy-amp.5907/), which has a SNR of 120 dB "Using near max power."

I wonder if there is a a reason for difference in specifications between 1 watt and 5 watts and full power (or 2V out), also 8 ohms and 4 ohms?
I don't see any replies addressing the part of your question about A-weighting, so I will give it a shot. A-weighting measurements use a filter that discounts noise components at frequencies where human hearing is less sensitive. So this results in a higher SNR figure than unweighted.
https://en.wikipedia.org/wiki/A-weighting
For the C268, the SNR specification (A-weighted @1W into 8 ohms) is 98dB. If you look at Amir's unweighted 8-ohm THD+N curve at 1W (where noise still dominates) you see that the value is 86dB. So the A-weighting makes a big difference.

If you want to use SNR numbers to try to guess whether you will hear noise from your speakers, I think the A-weighted numbers are more approriate (Maybe some here disagree?). For the C268 the unweighted measurement would suggest you can hear hiss from 90dB speakers at 1 meter. But I would bet (trusting NAD's spec) that you have to put your ear really close to the speaker to hear anything.
 
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#72
Seeing this review certainly makes me feel better about my recent purchase of a (refurbished) new NAD C368. The unit arrived spotless and still had the protective film on it. The remote would occasionally lock up, so I wonder if that's why it was returned. The seller quickly sent me a replacement remote and all is good in the world.

Am I satisfied with my experience? Absolutely. I am running TOSlink from my PC into the C368 and the sound is stellar. Balanced, great stereo imaging, and I can go far above comfortable listening levels without distortion. Looks like NAD got it right with these new amps.
 

DualTriode

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#73
There a couple CROWN COMTECH 210’s sitting here on my bench. One is connected to the APx555 also on the bench.



At 5 watts into 8R test resistors.



SINAD ch 1 is 92dB



SINAD ch 2 is 94dB



Both better than 90% of the amps on the list.



Attached is the FFT at 5 watts into 8R test resistors.



These old school amplifiers were less than $200 each.



Thanks DT
Crown Comtech 210 FFT 6 28 2020.png
 

DualTriode

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#74
Hello,

About the A weighted filter. The A-weighted filter is intended to sort of follow the sensitivity of human hearing at low levels. Single frequencies that stand out above the A-Weighted curve are outside the Standard and do not meet the requirements of using the weighting system. An example; many local building Departments call out a A-weighted number as a maximum loudness for say a Air Conditioning Unit on your prosperity. The A-Weighted measurement may be within the city standard. Now say that there is a single pitch wine at 440Hz that aggravates the neighbor is it okay? In this case the A-Weighted standard is used outside its parameters.

In the case of audio power supply noise like the single frequency AC noise is out side the A-Weighted standard. A-Weighted filter measurement does not tell us how loud the 60Hz, 120Hz, 180Hz ….. series of tones will sound in your speakers or headphones. This is an incorrect use of A-Weight filtering and measurement. A-Weight measurements for audio amplifiers are largely for marketing purposes.

For FFT’s I much prefer to use dBv on the vertical scale rather than dBr. dBr is relative to the output fundamental test frequency. The 60hZ, 120hZ and …….. series measured in volts or dBv are much more meaningful it terms of can I hear the power supply series of tones in my headphones or speakers.

Thanks DT
 
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#75
Advice needed!

The manual on C268 says it is not recommended to drive speakers with impedance less than 8 ohms. But most of my speakers are 4ohms. Any real issue or danger to use 2 C268 on bridge mode to dirve 4 ohms speakers?
 

Absolute

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#76
For FFT’s I much prefer to use dBv on the vertical scale rather than dBr. dBr is relative to the output fundamental test frequency. The 60hZ, 120hZ and …….. series measured in volts or dBv are much more meaningful it terms of can I hear the power supply series of tones in my headphones or speakers
Can you show/explain this a little more so that stupid people (me) can understand how and why that's the case? :D
Thanks for the clarification about A-weighting!
 

VintageFlanker

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#77
Advice needed!

The manual on C268 says it is not recommended to drive speakers with impedance less than 8 ohms. But most of my speakers are 4ohms. Any real issue or danger to use 2 C268 on bridge mode to dirve 4 ohms speakers?
First, there's a big probability your speakers aren't "4Ω", neither 8, for that matter. Speakers impedance is a variable value depending on frequency. Manufacturers spec their products in a very misleading way: "nominal impedance", being often far from the real minimal impedance measured. Knowing which speakers you're talking about would be useful to find any measurements of them out there.
But, let's face it: I don't know any consumer speakers having at least 8Ω minimal impedance.

Not allowed. Minimum impedance is 8 ohm in bridged mode.
Anyway, being honest, I really don't see the point to get two of these NADs (2X 900$) when amps such as Ncore NC502MP or dual NC500MP exist... :rolleyes: Cheaper, cleaner, and not limited with under-8Ω impedance.
 
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#78
First, there's a big probability your speakers aren't "4Ω", neither 8, for that matter. Speakers impedance is a variable value depending on frequency. Manufacturers spec their products in a very misleading way: "nominal impedance", being often far from the real minimal impedance measured. Knowing which speakers you're talking about would be useful to find any measurements of them out there.
But, let's face it: I don't know any consumer speakers having at least 8Ω minimal impedance.


Anyway, being honest, I really don't see the point to get two of these NADs (2X 900$) when amps such as Ncore NC502MP or dual NC500MP exist... :rolleyes: Cheaper, cleaner, and not limited with under-8Ω impedance.
Thank you for reply. I can get two about $1200 plus tax. My speakers are mainly diy speakers, Apollo MTM, and Helix MT from diysoundgroup, and Samba MT from Parts Express. Now I'm using topping E30 and Emotiva A-300 to drive them. I want to try class D. So that's why I'm consider 2 nad c268 as good option at the that price.
 

VintageFlanker

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#79
I want to try class D. So that's why I'm consider 2 nad as good option at the that price.
Unfortunately, that's not a good option at the price. This is old UcD Hypex design (that still perfoms well, according to @amirm measurements). You may get next generation, and better performing Ncore amps for less. Depending of where you live, you could get March Audio P502, VTC NC502, Hattor NC500 monoblocs, Audiophonics HPA-S500NC, and so on... All are performing better, though cheaper, than two C268 bridged and don't have the minimum-8Ω-concern.:cool:
 

DualTriode

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#80
Can you show/explain this a little more so that stupid people (me) can understand how and why that's the case? :D
Thanks for the clarification about A-weighting!
Hello,

Hello this topic is not really too difficult, it does have a degree of complexity.

If this was in my classroom we would have a discussion, a homework assignment and a quiz next week.

dB’s are kind of sort of logarithmic. dB =20 * Log x. If you want to convert 1volt to dB’s, dBV = Log . At 1 volt = 20 * Log (1) = 0dBV.

Homework; convert 0.000001v to dBV.

20 * Log (0.000001) = -120dBV. This is converting volts to dB. The dBV is relative to volts.

My preference is to use the dBV scale and let the test frequency peak go where it goes on the dBV scale. I like to keep the noise floor constant in terms of real volts so I have an idea of what the power supply noise of 60 Hz, 120Hz, 180Hz, and ….. Hz you know the power supply series of tones thing, will be at the speaker input.

Here at ASR we measure the output scale relative to 5 watts output. Power out is calculated from the voltage measured across a 4 Ohm test resistor. P = volts X volts / 4 ohms. At 5 watts into a 4ohm load the voltage measured across 4 ohms is 4.4721 volts. On the dBr scale where the output is relative to 5 watts, the test frequency peak on the dBr FFT plot is = 0dBr not 12.91db as it would read on the dBV scale.

At 5 watts output the dBr scale is shifted down 12.91dB compared to the dBV scale.

Thanks DT
 
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